Hey, I think all this work has been done before - and is under a GPL on CodeCogs.

Your code below essentially does the same as this component: http://www.codecogs.com/d-ox/units/time/gmtoffset.php

To figure out Summertime, you need to look at the dates. There are several different formulas used around the world to decide when summer time starts/ends. Look at http://www.codecogs.com/d-ox/units/time/summertime.php

The offset between any main location are given in: http://www.codecogs.com/d-ox/units/time/worldtimediff.php

What else did you need?

Will.